--- title: Sensor Tutorial with C/C++ (Arduino/Wiring) and Blues Cygnet description: Build an MCU-controlled application that reads from an external sensor and sends readings to the Notecard to start building your IoT application using Cellular, Satellite, LoRa, or WiFi connectivity. source_url: https://dev.blues.io/guides-and-tutorials/collecting-sensor-data/ canonical_url: https://dev.blues.io/guides-and-tutorials/collecting-sensor-data/ markdown_url: https://dev.blues.io/guides-and-tutorials/collecting-sensor-data.md --- # Collecting Sensor Data C/C++ (Arduino/Wiring) and Blues Cygnet *Don't see your favorite hardware here? Rest assured the Notecard works with virtually every MCU and SBC available. If you can't figure out how to complete this tutorial [let us know in our forum](https://discuss.blues.com/) and we can help you out.* ## Introduction *This tutorial should take approximately 40-50 minutes to complete.* In this tutorial, you'll learn how to take sensor data, queue it on your Notecard, and synchronize it to Blues Notehub. You'll use **C/C++ (Arduino/Wiring)** running on a **Blues Cygnet**. If you would like to use a different language or board, modify the dropdowns at the top of this guide. And if you get stuck at any point, feel free to reach out for help in our [community forum](https://discuss.blues.com). This tutorial uses mock sensor readings for simplicity, but feel free to hook up a physical sensor of your choice and use that instead. The goal of this tutorial is to demonstrate reusable firmware techniques you can apply to your own Notecard-based projects. ## Set up Hardware > **Note:** > > This tutorial assumes you have already wired your Notecard to your host. If you haven't, complete the [Host Wiring Guide](https://dev.blues.io/guides-and-tutorials/host-wiring-guide.md) before continuing. In order to complete this guide, you'll need the following: - A Blues Cygnet. - A USB-C cable. > **Note:** > > If you’re following the **Feather Kit for Cell+WiFi Quickstart**, all you need to do at this point is plug your Cygnet into the Feather headers on the Notecarrier F, then connect it to your computer using the **USB-C port on the Cygnet**. > > ![The Cygnet connected to a Notecarrier F](https://dev.blues.io/images/guides/first-sensor/notecarrier-f/carr-f-cygnet.jpg?v=0e4b4745) ## Create a Notehub Project Now that your hardware is all connected, let's create a new Notehub project to receive sensor readings from your Notecard. 1. Navigate to [notehub.io](https://notehub.io) and log in, or create a new account. 2. Click the **Create Project** button. 3. In the New Project dialog, give your project a name and ProductUID. ![How to create a new Notehub project](https://dev.blues.io/images/guides/notehub/create-project.png?v=bf29a47a) > **Note:** > > The ProductUID must be globally unique, so we recommend a namespaced name like `"com.your-company.your-name:your_product"`. 4. Take note of your ProductUID. This identifier is used by Notehub to associate your Notecard with your project. ![Where to find your product UID](https://dev.blues.io/images/guides/notehub/product-uid.png?v=d829ab1d) ## Write Firmware > **Tip:** > > This tutorial shows how to write host firmware by hand so you can understand the fundamentals. In practice, we recommend using an AI coding assistant with Blues Expert MCP for development. Blues Expert MCP connects your AI coding assistant (Claude Code, GitHub Copilot, Cursor) directly to our API docs, providing live request validation and firmware best practices for Arduino, C, Zephyr, and Python. [Install the Blues Expert MCP →](https://dev.blues.io/tools-and-sdks/generative-ai-tools/blues-expert-mcp.md) For this portion of the guide you'll be writing firmware on your Blues Cygnet that will communicate with the Notecard. You can write the firmware using either PlatformIO (recommended) or Arduino IDE. ### Communicating With Your Notecard When communicating with the Notecard, the easiest approach is to use one of [Notecard's official SDKs](https://dev.blues.io/tools-and-sdks/firmware-libraries.md). Since this tutorial uses Arduino, let's install the Arduino SDK, [note-arduino](https://github.com/blues/note-arduino). **Install the Notecard Arduino Library** **Arduino IDE** 1. To use the [note-arduino library](https://github.com/blues/note-arduino), you'll need to add it to the Arduino IDE. 2. Click on **Tools > Manage Libraries...** 3. Search for "Blues" in the input box and click the "Install" button next to the "Blues Wireless Notecard" result. ![Installing the Blues Wireless Notecard library.](https://dev.blues.io/images/guides/first-sensor/arduino/install-library.png?v=ea1aec79) 4. Create a new sketch and select the **Sketch > Include Library > Contributed Libraries > Blues Wireless Notecard** menu option, to add the following include to your sketch: ```cpp #include ``` **PlatformIO** 1. Open PlatformIO within Visual Studio Code. 2. Click the **Projects & Configuration** menu, and hit the **Create New Project** button. 3) Give the project a **Name**, select a board of **Blues Cygnet**, and click **Finish**. ![Creating a new PlatformIO project](https://dev.blues.io/images/guides/first-sensor/arduino/platformio-project-cygnet.png?v=2ab5789d) 4) Open your new project's `platformio.ini` file, and replace its contents with the default Cygnet configuration below, which includes the Blues Notecard library in its `lib_deps`. ```c [env:blues_cygnet] platform = ststm32 board = blues_cygnet upload_protocol = stlink framework = arduino build_flags = -D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC monitor_speed = 115200 lib_deps = Wire blues/Blues Wireless Notecard ``` > **Warning:** > > The above configuration assumes you're connecting to your Cygnet via an STLink programmer like the [STLINK-V3MINI](https://shop.blues.com/products/stlink-v3mini?utm_source=dev-blues\&utm_medium=web\&utm_campaign=store-link). If you instead intend to program your Cygnet using only a USB cable, you'll need to set your `upload_protocol` to `dfu` and follow these instructions to force the Cygnet to jump into its bootloader: > > Press and hold the `BOOT` button on the Cygnet, press and release `RESET`, then release `BOOT` every time you want to upload firmware. 5. Open your project's `main.cpp` file, and paste the following include under the existing `` include. Leave your `main.cpp` file open for the steps below. ```c #include ``` **Set Up Your Notecard** Select the tab matching the interface you wired up in the [Host Wiring Guide](https://dev.blues.io/guides-and-tutorials/host-wiring-guide.md), then follow the steps for your chosen interface. > **Note:** > > If you're using a Notecarrier CX or Notecarrier F, you're likely using I2C, as those boards connect the host to Notecard over I2C by default. **I2C** 1. At the top of your sketch, define the Serial interface you'll use for logging. ```cpp #define usbSerial Serial ``` 2. Add a definition for your ProductUID using the value you specified when creating your Notehub project. ```cpp #define productUID "com.your-company.your-name:your_product" ``` 3. Above the `setup()` and `loop()` functions, declare a **global** object to represent the Notecard. ```cpp Notecard notecard; ``` 4. In the `setup()` function, initialize the `usbSerial` object. The short delay gives boards with native USB time to enumerate before the first log line is written. ```cpp delay(2500); usbSerial.begin(115200); ``` 5. Initialize the I2C connection to the Notecard using the `notecard.begin()` function. Then, use `setDebugOutputStream()` to link the debug output to `usbSerial` with the following code: ```cpp notecard.begin(); notecard.setDebugOutputStream(usbSerial); ``` **UART** 1. At the top of your sketch, define the Serial interfaces you'll use for logging and Notecard communication. ```cpp #define usbSerial Serial #define txRxPinsSerial Serial1 ``` 2. Add a definition for your ProductUID using the value you specified when creating your Notehub project. ```cpp #define productUID "com.your-company.your-name:your_product" ``` 3. Above the `setup()` and `loop()` functions, declare a **global** object to represent the Notecard. ```cpp Notecard notecard; ``` 4. In the `setup()` function, initialize the `usbSerial` object. ```cpp usbSerial.begin(115200); while (!usbSerial) { ; // wait for serial port to connect. Needed for native USB } usbSerial.println("Starting..."); ``` 5. Initialize the UART connection to the Notecard using the `begin()` function. Then, use `setDebugOutputStream()` to link the debug output to `usbSerial` with the following code: ```cpp notecard.begin(txRxPinsSerial, 9600); notecard.setDebugOutputStream(usbSerial); ``` 6) Now, we'll configure the Notecard. Using the `hub.set` request, we associate this Notecard with the ProductUID of your project and set the Notecard to operate in `continuous` mode, which indicates that the device should immediately make a connection to Notehub and keep it active. ```cpp { J *req = notecard.newRequest("hub.set"); if (req != NULL) { JAddStringToObject(req, "product", productUID); JAddStringToObject(req, "mode", "continuous"); notecard.sendRequest(req); } } ``` The lines above build up a JSON object by adding two string values for product and mode, and then fire the request off to the Notecard with the `sendRequest()` function. **Flash Your Firmware** 1. If you're NOT using an STLink programmer like the [STLINK-V3MINI](https://shop.blues.com/products/stlink-v3mini?utm_source=dev-blues\&utm_medium=web\&utm_campaign=store-link), before you upload your compiled sketch to the Blues Cygnet you'll need to enter bootloader mode by pressing and holding the `BOOT` button, pressing `RESET`, then releasing both buttons. **Arduino IDE** If you're using Arduino IDE, click the Upload button (right arrow icon) to flash the firmware to your device. ![Uploading firmware from Arduino IDE](https://dev.blues.io/images/guides/first-sensor/feather/arduino-tutorial-upload.png?v=cc42f1f4) **PlatformIO** If you're using PlatformIO, click the Upload button (right arrow icon) at the bottom of the VS Code interface to flash the firmware to your device. ![Uploading firmware from PlatformIO](https://dev.blues.io/images/guides/first-sensor/feather/platform-io-upload.png?v=ce582853) **View the Serial Log** Now that your firmware is running on your device, you can use a Serial Monitor to view device output. If you are using an STLINK programmer, you will need to connect a second USB cable from your computer directly to your STM32-based host to view serial output. Complete the instructions below in your preferred IDE. **Arduino IDE** 1. If you're using Arduino IDE, open the Arduino Serial Monitor. If everything has been connected and configured properly, you'll see a few debug messages, including the JSON object you sent, as well as the response from the Notecard `{}`. **NOTE:** You may need to press the `RESET` button on your board or upload your code a second time to see the result of the `hub.set` request, as the Serial Monitor only captures output when it is actively running. ![Serial Monitor hub.set response](https://dev.blues.io/images/guides/first-sensor/feather/serial-configured.png?v=38df04b5) **PlatformIO** 1. If you're using PlatformIO, open the Serial Monitor by clicking the plug icon in the bottom of VS Code. ![PlatformIO Serial Monitor icon](https://dev.blues.io/images/guides/first-sensor/feather/platform-io-serial-monitor.png?v=73e9267a) After selecting the appropriate serial port, you should see a few debug messages, including the JSON object you sent and the response from the Notecard `{}`. **NOTE:** You may need to press the `RESET` button on your board or upload your code a second time to see the result of the `hub.set` request, as the Serial Monitor only captures output when it is actively running. ![PlatformIO terminal Notecard response](https://dev.blues.io/images/guides/first-sensor/feather/platform-io-serial-output.png?v=5d8382e8) ### Read from the sensor Now that you've configured your Arduino to communicate with the Notecard, let's grab some sensor readings. > **Note:** > > If you have your own sensor, feel free to hook it up and use your own values instead of this tutorial's mocked ones. **Install the NotecardPseudoSensor Library** To generate mock sensor readings you'll use the [NotecardPseudoSensor library](https://github.com/blues/notecard-pseudo-sensor). Complete the instructions below for your preferred IDE. **Arduino IDE** 1. To add the library to Arduino IDE start by clicking on **Tools > Manage Libraries...** 2. Search for "NotecardPseudoSensor" in the input box and click the **Install** button next to the "Blues Wireless Notecard Pseudo Sensor" result. ![NotecardPseudoSensor library install](https://dev.blues.io/images/guides/first-sensor/arduino/pseudo-sensor-library.png?v=929defbe) **PlatformIO** 1. Click the **Libraries** menu within PlatformIO. 2. Search for "Notecard" and scroll a bit to find the NotecardPseudoSensor library. ![Finding the pseudo sensor library in PlatformIO](https://dev.blues.io/images/guides/first-sensor/feather/platform-io-pseudo-library.png?v=d2302e56) 3. Click the "Add to Project" button and select your project from the dropdown list provided. This will add the library to the `lib_deps` section of your project's `platformio.ini` file. ![Adding the pseudo sensor library in PlatformIO](https://dev.blues.io/images/guides/first-sensor/feather/platform-io-add-to-project.png?v=bd9e1875) **Gather Sensor Readings** 1. Add the following include to the top of your sketch: ```cpp #include ``` 2. Next, include the following `namespace` under your includes. ```cpp using namespace blues; ``` 3. After that, create an instance of `NotecardPseudoSensor` with the line of code below. Place this directly under your existing `Notecard notecard` statement. ```cpp NotecardPseudoSensor sensor(notecard); ``` 4. Finally, place the following code in your `loop` function, which generates mock temperature and humidity readings, prints them to the console, and then waits 15 seconds before exiting the loop. ```cpp float temperature = sensor.temp(); float humidity = sensor.humidity(); usbSerial.print("Temperature = "); usbSerial.print(temperature); usbSerial.println(" *C"); usbSerial.print("Humidity = "); usbSerial.print(humidity); usbSerial.println(" %"); delay(15000); ``` 5. Upload this code to your Blues Cygnet. Open the Serial Monitor and you'll see temperature and humidity readings every 15 seconds. ![Serial monitor temperature and humidity](https://dev.blues.io/images/guides/first-sensor/arduino/arduino-tutorial-serial-monitor.png?v=09c9c2c5) ### Send Sensor Readings to the Notecard Now that we're getting sensor readings, let's send these to our Notecard. 1. To send a sensor reading to the Notecard, we'll need to construct a new JSON request to the `note.add` API that includes a new Notefile name (`sensors.qo`), sets the `sync` field to true to instruct the Notecard to sync to Notehub immediately, and finally, sets the `body` to the sensor temperature and humidity. Add the following in `loop` right after the `usbSerial` commands to print out readings. ```cpp { J *req = notecard.newRequest("note.add"); if (req != NULL) { JAddStringToObject(req, "file", "sensors.qo"); JAddBoolToObject(req, "sync", true); J *body = JAddObjectToObject(req, "body"); if (body) { JAddNumberToObject(body, "temp", temperature); JAddNumberToObject(body, "humidity", humidity); } notecard.sendRequest(req); } } ``` 2) Upload this code to your device. After reboot, the Serial monitor will update to display the response from the `note.add` request (the total number of Notes in the notefile) each time you add a new reading. ![Serial Monitor note totals output](https://dev.blues.io/images/guides/first-sensor/feather/notes-mock-lib.png?v=5ec543e1) > **Note:** > > If you're using a Notecard for LoRa to complete this tutorial you have one additional step. Because the Notecard for LoRa requires a template for every Notefile you use, you must define a template for the `sensors.qo` Notefile in your `setup()` function. You can add the following code to the bottom of your `setup()` function to fix the problem. > > ```cpp > { > J *req = notecard.newRequest("note.template"); > if (req != NULL) { > JAddStringToObject(req, "file", "sensors.qo"); > JAddNumberToObject(req, "port", 1); > JAddStringToObject(req, "format", "compact"); > J *body = JCreateObject(); > if (body != NULL) { > JAddNumberToObject(body, "temp", 14.1); > JAddNumberToObject(body, "humidity", 14.1); > JAddItemToObject(req, "body", body); > notecard.sendRequest(req); > } > } > } > ``` > > Learn more about Notefile templates in [Working with Note Templates](https://dev.blues.io/notecard/notecard-walkthrough/low-bandwidth-design.md#working-with-note-templates). > **Note:** > > Concerned about the size of `note-arduino`? You can [communicate with the Notecard without using the library](https://dev.blues.io/guides-and-tutorials/notecard-guides/notecard-communication-without-a-library.md). ## View Data in Notehub Once you start capturing readings, your Notecard will initiate a connection to Notehub and will start transferring Notes. Depending on signal strength and coverage in your area, it may take a few minutes for your Notecard to connect to Notehub and transfer data. 1. Return to [notehub.io](https://notehub.io) and open your project. You should see your Notecard in the Devices view. ![The new device in Notehub](https://dev.blues.io/images/guides/notehub/new-device.png?v=f506b425) > **Note:** > > Each Notecard has a factory-assigned, globally unique identifier known as a [DeviceUID](https://dev.blues.io/api-reference/glossary.md#deviceuid). Notehub uses this identifier in the Devices view by default (for example, `dev:868531061604976` in the screenshot above). > > If you’d prefer to use your own identifier—such as a human-readable name or an internal ID—you can assign a [serial number](https://dev.blues.io/api-reference/glossary.md#product-sn) to your Notecard in one of the following ways: > > - **In Notehub:** Double-click your device in the Devices view to open its details, where you can edit the serial number. > - **Via the Notehub API:** Set the reserved `_sn` environment variable using the [Set Device Environment Variables](https://dev.blues.io/api-reference/notehub-api/device-api.md#set-device-environment-variables) endpoint. > - **Via the Notecard API**: Include an `sn` argument in the [hub.set request](https://dev.blues.io/api-reference/notecard-api/hub-requests/latest.md#hub-set) you used to configure your Notecard. 2. Now, click on the Events left menu item. Once your sensor Notes start syncing, they'll show up here. You may need to refresh the page to see newly synced Notes. ![The event list in Notehub](https://dev.blues.io/images/guides/notehub/events.png?v=76323245) ## Use Environment Variables [Environment variables](https://dev.blues.io/guides-and-tutorials/notecard-guides/understanding-environment-variables.md) are a Notehub state and settings management feature that allow you to set variables in key-value pairs, and intelligently synchronize those values across devices and fleets of devices. In this section you'll learn how environment variables work by creating a variable that determines how often your firmware should take sensor readings. ### Using Environment Variables in Firmware The Notecard provides [a set of requests for working with environment variables](https://dev.blues.io/api-reference/notecard-api/env-requests.md). The most common of these requests is [`env.get`](https://dev.blues.io/api-reference/notecard-api/env-requests.md#env-get), which allows you to retrieve the value of an environment variable. Complete the steps below to use the `env.get` request to retrieve and use the `reading_interval` environment variable. 1. In `setup()`, adjust your existing `hub.set` configuration to set the `inbound` argument to `5`. This tells your Notecard to look for inbound changes from Notehub every 5 minutes. ```c { J *req = notecard.newRequest("hub.set"); if (req != NULL) { JAddStringToObject(req, "product", productUID); JAddStringToObject(req, "mode", "continuous"); JAddNumberToObject(req, "inbound", 5); // ADD THIS LINE notecard.sendRequest(req); } } ``` 2. Next, place the following new function at the bottom of your sketch. ```c // This function assumes you’ll set the reading_interval environment variable to // a positive integer. If the variable is not set, set to 0, or set to an invalid // type, this function returns a default value of 60. int getSensorInterval() { int sensorIntervalSeconds = 60; J *req = notecard.newRequest("env.get"); if (req != NULL) { JAddStringToObject(req, "name", "reading_interval"); J* rsp = notecard.requestAndResponse(req); int readingIntervalEnvVar = atoi(JGetString(rsp, "text")); if (readingIntervalEnvVar > 0) { sensorIntervalSeconds = readingIntervalEnvVar; } notecard.deleteResponse(rsp); } return sensorIntervalSeconds; } ``` 3. Then, add this line after the `#include` lines at the top of the file. ```c int getSensorInterval(); ``` 4. Next, find the `delay(15000)` line at the bottom of your `loop()` function, and replace it with the code below. ```c int sensorIntervalSeconds = getSensorInterval(); usbSerial.print("Delaying "); usbSerial.print(sensorIntervalSeconds); usbSerial.println(" seconds"); delay(sensorIntervalSeconds * 1000); ``` > **Note:** > > Notecard for LoRa requires a template for each environment variable you use. If you're using a Notecard for LoRa to complete this tutorial, add the code below to your `setup()` to provide a template for the `reading_interval` variable. > > ```c > { > J *req = notecard.newRequest("env.template"); > if (req != NULL) { > J *body = JCreateObject(); > if (body != NULL) { > JAddNumberToObject(body, "reading_interval", 21); > JAddItemToObject(req, "body", body); > notecard.sendRequest(req); > } > } > } > ``` > > Here `21` is the type hint for a 1-byte unsigned integer (`0`–`255`). If your `reading_interval` may exceed 255, use `22` (a 2-byte unsigned integer) instead. With this code in place, your firmware now uses the `reading_interval` environment variable to determine how many seconds to delay in between sensor readings. If you flash this updated code to your device and open your serial monitor, you can see the device using the default value for `reading_interval` of 60 seconds. ### Setting an Environment Variable Now that we have our device programmed to retrieve an environment variable from Notehub, we will create that variable. Environment variables can be set in the Notehub UI or through the Notehub API. In this tutorial you'll learn how to set the values through the Notehub UI. If you'd like to instead set environment variables through the Notehub API, refer to environment variable requests in the [Project API](https://dev.blues.io/api-reference/notehub-api/project-api.md). 1. Return to your Notehub project, go to the **Devices** page, and double click your device. You should see a screen that looks like this. ![The Notehub device screen](https://dev.blues.io/images/guides/first-sensor/environment/device-screen.png?v=cdb91b6c) 2. Click the **Environment** tab. 3. Under the **Device environment variables** header, define a new environment variable named `reading_interval` and set its value to `30`. ![The environment screen with a new value set](https://dev.blues.io/images/guides/first-sensor/environment/setting-a-variable.png?v=e576ef35) Now that you have an environment variable set, you'll see it reflected on your device **after your configured `inbound` interval has passed**. ![The environment screen with a new value set](https://dev.blues.io/images/guides/first-sensor/environment/env-var-serial-log.png?v=123d9606) > **Note:** > > On cellular and WiFi-based Notecards you can use the [`hub.set` request](https://dev.blues.io/api-reference/notecard-api/hub-requests/latest.md#hub-set)'s `sync` argument to immediately receive inbound updates instead of relying on the `inbound` interval. And with that, you've used your first environment variable on your Notecard! To see the real power of environment variables in action, try returning to Notehub and updating your device's `reading_interval` with your serial monitor open. > **Note:** > > This tutorial covers several configuration settings that are best used when you have your Notecard connected to mains power. > > - In the `hub.set` request, setting `mode` to `"continuous"` tells the Notecard to maintain an active network connection. > > - In the `hub.set` request, the `sync` argument (when set to `true`) tells the Notecard to immediately synchronize inbound Notes and environment variables from Notehub. > > - In the `note.add` request, setting `sync` to `true` tells the Notecard to immediately synchronize all outbound Notes to Notehub. > > Because each of these settings causes the Notecard to use more power, you may wish to disable them if you plan to transition your project to battery power. You can run the command below to put your Notecard into `periodic` mode with the `sync` argument turned off. > > ```json > { > "req": "hub.set", > "mode": "periodic", > "sync": false, > "outbound": 60, > "inbound": 120 > } > ``` > > For a deeper look at how these settings work together, watch [An In-Depth Guide to Notecard’s hub.set Request](https://www.youtube.com/watch?v=3D2p1t8UMHQ). The video steps through complete configuration scenarios that show how your choice of `mode`, `outbound`, `inbound`, and `sync` values determines exactly when your Notecard connects and syncs with Notehub. ## Next Steps **Congratulations!** You've successfully connected your Blues Cygnet to your Notecard and built a basic IoT project. If you're following a Cell+WiFi Quickstart, next we recommend learning how to send (and visualize) your data in a cloud application: 1. ~~Use the Notecard to Send Data~~ 2. ~~Set Up Your Microcontroller~~ 3. ~~Build Your First IoT App With Blues~~ 4. [Send Data to Your Cloud](https://dev.blues.io/guides-and-tutorials/routing-data-to-cloud.md) At any time, if you find yourself stuck, please reach out on the [community forum](https://discuss.blues.com/).